Patent GB-1526207-A (GB)
Title: Apparatus suitable for use in performing hf operations on workpieces
IPC: H05B6/50
Filed: 1974-12-20
Abstract: 1526207 Microwave heating BRITISH UNITED SHOE MACHINERY CO Ltd 31 Oct 1975 [20 Dec 1974] 55256/74 Headings H3W, H5H and H3F [Also in Division B5] Apparatus for performing an HF operation on a workpiece W comprises a resonant cavity 22 within which pressure can be applied by platens 26, 32 to a workpiece W; a triode valve 20, the anode of which is connected to the interior of the resonant cavity 22 by a capacitor 42; variable tuning means 46 connected between the grid of the valve 20 and a feedback coil 44 contained within the cavity, the means 46 acting to tune the grid circuit of the valve 20 to a particular current; voltage applying means for applying a first, lower, voltage or a second, higher, voltage to the valve 20; and operating means operable, while the first voltage is applied to the valve 20, to vary the means 46 to cause the grid current of the valve 20 to increase and an HF field to build up in the resonant cavity 22, the voltage applying means comprising switching means operable upon the grid current attaining a certain value to replace the first voltage by the second voltage. The HF operation is a welding operation, i.e. the securing together by heat and pressure of two parts of weldable material or two parts of non-weldable material with a heat-activated adhesive layer therebetween, or an embossing operation, i.e. providing the surface of a workpiece with a desired pattern by heat and pressure. As shown, an A.C. mains supply is connected to three contacts A on a circuit breaker 10 connected to an auto transformer 12, a contact box 14, a transformer 16 and a rectifier 18. The transformer 16 may receive either full mains voltage or a lower voltage, e.g. 40% of the mains voltage. In the illustrative apparatus, the output of the rectifier 18 is either 2 or 5 kilovolts according to whether the transformer 16 receives the lower or full mains voltage, respectively. The platen 26 depends from a rod 28 and the cavity 22 is formed between a cylindrical sheet copper wall 29 mounted on the platen 26 and outer sheet copper walls 31 depending from a rectangular sheet copper top-piece 30. The raisable and lowerable lower platen 32 is mounted on a piston rod 36 of an hydraulic cylinder. A contact member 38 mounted on the walls 31 is urged by springs 40 into contact with the platen 32 in the upper position thereof. The variable tuning means 46 comprises a capacitor having two plates each constructed of semicircular vanes. The capacitance of the capacitor 46 is variable by rotation of one of the plates about a central axis thereof, this movable plate being mounted on an output shaft 49 of a motor 48 operable when it receives an input from an amplifier 58. The shaft 49 carries cams 50, 52 arranged to operate microswitches 54, 56 connected to a relay-operated switch 57. The amplifier 58 has two supply lines originating at contacts B, C, a signal line and an earth line, and only receives a supply when one of the microswitches 54, 56 is made and that microswitch is switched in by the switch 57. If the signal line supplies a positive voltage to the amplifier 58, the capacitance is increased but is decreased if this line supplies a negative voltage. If no voltage is supplied by the signal line, the capacitance is kept constant. The grid of the valve 20 is connected to resistors 62, 64, the resistor 64 being connected to the earth line of the amplifier 58. The grid current of the valve 20 is sensed by a sensor 66 which operates a relay 68 when this current reaches a predetermined value, in this case 200 milliamps. A voltage proportional to the grid current is supplied by a connection to a contact on the resistor 64 to a resistor 70 which also receives a reference D.C. voltage from a transformer and rectifier 72. A contact of the resistor 70 is connected to a switch 60 which can operate either to pass the voltage received from the resistor 70 to the signal line of the amplifier 58 or to connect the amplifier 58 to a contact E supplying a fixed voltage, both positions of the switch 60 causing operation of the motor 48. The cathode of the valve 20 is powered by a transformer 74. A transformer 76 powers a relay box 78 which operates the contacts in the box 14, the switch 57, the switch 60, and two timer circuits 80, 82 connected to the box 78. At the beginning of each operation, the cavity 22 is open, the microswitch 56 is switched in by the switch 57, the switch 60 connects the contact E to the amplifier 58, and neither set of contacts in the box 14 is made. If the capacitor 46 is not at a predetermined datum value, the motor 48 is operated until this value is reached and the cam 52 operates the microswitch 56 to cut off the supply to the amplifier 58. The workpiece W is placed on the platen 32 which is raised to close the resonant cavity 22 and a pressure switch (not shown) is operated to cause (a) the lower voltage of 2 kilovolts to be applied to the valve 20, (b) the input to the transformer and rectifier 72 to be switched on, (c) a relay to operate the switch 60 to connect the resistor 70 to the amplifier 58, and (d) a relay to operate the switch 57 to switch out the microswitch 56 and switch in the microswitch 54. The motor 48 operates to increase the capacitance of the capacitor 46 and cause the grid current of the valve 20 to increase and an HF field to build up in the cavity 22. When the grid current reaches 200 milliamps, the sensor 66 causes the relay 68 to cause the 2 kilovolts applied to the valve 20 to be replaced by 5 kilovolts so that the HF field in the cavity 22 increases in power and the workpiece W is heated by dielectric heating. The motor 48 continues to operate to allow buildup of the grid current to a second predetermined value, in this case 700 milliamps, at which the signal received by the amplifier 58 from the resistor 70 is zero and the motor 48 stops. Should the grid current vary during the remainder of the period of application of the HF field, the motor 48 will operate to return the grid current to 700 milliamps. If the capacitance of the capacitor 46 cannot be raised sufficiently to bring the grid current to 700 milliamps, the capacitance is held at its maximum value by the cam 50 breaking the microswitch 54 so that the amplifier 58 receives no supply from the contacts B, C and the motor 48 is stopped. The timer 80, switched on by operation of the relay 68, switches off the HF field in the cavity 22 after a predetermined time interval and switches off the input to the transformer and rectifier 72 so that the amplifier 58 receives no signal and, thus, the motor 48 does not operate. The timer 80 energizes the timer 82 which allows the workpiece W to remain subjected to pressure by the platens 26, 32 for a predetermined period after the HF field has ceased. Upon opening of the cavity 22, the pressure switch is released to cause the switch 60 to connect the amplifier 58 and the contact E and the switch 57 to switch in the microswitch 56 so that the motor 48 operates to return the capacitor 46 to the datum value.